Electromagnetic motors are utilized in a variety of applications, both as motors to deliver force and as sensors to detect force acting on the motor. Electromagnetic motors employ one or more coils and one or more magnets that comprise an armature (moving portion of the motor) and a stator (stationary portion of the motor) to generate an output force. One type of electromagnetic motor, sometimes referred to in the art as a Lorentz motor, operates on the principle that force is created by the interaction of magnetic fields. In this case, a current is provided to a conductor, such as a coil, located in a magnetic field provided by a magnet, to generate a force that is proportional to the input current. In other words, a linear relationship exists between the current provided to the conductor and the output force generated by the motor. So long as the conductor remains in the area of the magnetic field, a linear current to force relationship exists. Unfortunately, however, in this design, flux from the magnetic field passing through the conductor increases the reluctance, e.g. the magnetomotive force across a structure divided by the flux through the area of the structure, of the magnetic path thereby increasing the size of the magnet required for a given output force.
Another type of electromagnetic motor, known as a variable reluctance motor, operates on the principle that an iron core placed in a magnetic field always aligns in the minimum reluctance position e.g. where the magnetic field meets the lowest resistance. In a variable reluctance motor, both the armature and the stator have iron cores with salient poles. Due to the salient poles in the armature and stator, displacement of the armature relative to the stator produces a variation of the reluctance of the magnetic circuit.
Operationally, the windings of the variable reluctance motor are excited by a complex time varying source, such that when a phase winding is energized, the armature positions itself to achieve a minimum reluctance for that phase. Just as the armature approaches equilibrium, current is switched to the next phase winding to maintain motion. Variable reluctance motors, while being capable of producing large forces in a compact form factor, produce these forces in a non-linear manner, as the reluctance is non-linear. In this regard, the force produced by a variable reluctance motor is substantially proportional to the product of the square of the winding ampere-turns and the rate of change of the inductance as a function of armature position.
In many applications, however, it is desirable that a motor produce a high force output in a compact form factor, respond linearly to current input, and be independent of the position of the armature relative to the stator. For instance, in the class of hearing aids generally referred to as implantable hearing aids, some or all of various hearing augmentation componentry is positioned subcutaneously on or within a patient's skull, typically at locations proximate the mastoid process. Implantable hearing aids may be generally divided into two sub-classes, namely semi-implantable and fully implantable. In a semi-implantable hearing aid, components such as a microphone, signal processor, and transmitter may be externally located to receive, process, and inductively transmit an audio signal to implanted components such as a transducer. In a fully-implantable hearing aid, all of the components, e.g. the microphone, signal processor, and transducer, are located subcutaneously. In either arrangement, an implantable transducer is utilized to stimulate a component of the patient's auditory system.
One type of implantable transducer is an electromechanical transducer having a driver to move a vibratory actuator. As will be appreciated, it is desirable that the driver output force be linearly related to the input current as controlled movements of the actuator are utilized to stimulate one or more bones of the ossicular chain to cause or enhance the sensation of hearing for the patient. It is also, desirable that the size of implantable transducers be minimized due to their environment of use.